Azeotropic distillation of hydrocarbon mixtures



Patented July 26, 1949 AZEOTROPIC DISTILLATION OF HYDROCARBON RIIXTURES George R. Lake and Josephine M.

Beach, Calif., assignors to Union Los Angeles, Calif., a corporation of California, of California No Drawing. Application Stribley, Long Oil Company August 21, 1944,

Serial No. 550,479 11 Claims. (Cl. 202-42) This invention relates to a process of azeotropic distillation to prepare pure hydrocarbons from complex petroleum fractions which are dimcult to separate by ordinary fractional distillation due to small differences in boiling points existing in the hydrocarbons contained in the fraction and is a continuation-in-part of our copending application, Serial No 550,381, now abandoned.

The process of separating one hydrocarbon component from another hydrocarbon component of substantially the same boiling point, contained in a complex hydrocarbon fraction, by azeotropic distillation is well known. This process consists of distilling the hydrocarbon fraction in the presence of an extraneous substance which has a preferential afllnity for one of the hydrocarbon components contained in the fraction, thus causing a disturbance of the vapor pressure equilibrium that formerly existed in the fraction in such a manner that the partial vapor pressure or fugacity of at least one component of the fraction is changed sufllciently to permit its separation by controlled fractional distillation. Heretofore in such an azeotroping process there has been effected the separation of the relatively more parafiinic hydrocarbons with the extraneous substance leaving as undistilled bottoms relatively more aromatic hydrocarbons which may or may not contain a portion of the extraneous substance. In the present description of our invention the aforesaid type of distillation will hereinafter be referred to as azeotropic distillation, and the overhead product consisting of the extraneous substance together with the component or components most affected by said extraneous substance will hereinafter be referred to as the azeotropic distillate, and the residue remaining as bottoms from the azeotroplc distillation will hereinafter be referred to as azeotroplc bottoms. For example, in separating an aromatic hydrocarbon from a hydrocarbon fraction containing as well olefinic, paraiilnic and naphthenic hydrocarbons it has been the practice to add to such a fraction an extraneous substance such as for example acetone or methyl ethyl ketone which has the eflect of increasing the vapor pressure of the relatively more paraflinic hydrocarbons. Thus by distilling a hydrocarbon fraction containing benacne in the presence of acetone it is possible to take overhead as the azeotropic distillate an azetrope of acetone and the non-aromatic hydrocarbons present in said hydrocarbon fraction, leaving substantially all of the benzene as azeo tropic bottoms. It is frequently necessary to acid treat and redistill the azeotroplc bottoms in order to obtain essentially pure benzene. The preparation of pure toluene has been accomplished in a similar manner using azeotrope formers of different properties, as for example methyl ethyl ketone. Such a process has many disadvantages familiar to those skilled in the art. Due to the fact that aromatics are usually present in minor proportions in petroleum fractions, it has been necessary in a majority of cases when employing a. process as described above to handle excessive amounts of material in order to obtain relatively small amounts of aromatic hydrocarbons. Another disadvantage inherent in such a process is the necessity of redistilllng the azeotropic bottoms to obtair aromatics in essentially pure form; thus, for the recovery or removal of an aromatic hydrocarbon contained in a petroleum fraction in the amount of about 5%, it has been necessary in the conventional method of azeotropic distillation to distill overhead of the hydrocarbon fraction together with a suillcient amount of an azeotrope former to effect thenecessary boiling point depression of this 95% men-aromatics contained in the fraction.

It is an object of our invention to prepare pure hydrocarbons in a new and novel manner from petroleum fractions which are impossible of separation by means of ordinary fractional distillation. It is a further object of our invention to effect this separation by a method in which distillation and extraction loads are greatly reduced.

A more specific object of our invention is to employ an azeotrope former having the reverse effect of those heretofore employed thus decreasing the boiling point of the relatively aromatic hydrocarbons permitting their recovery in the azeotrope distillate rather than in the azeotrope bottoms and to leave the relatively non-aromatic hydrocarbons as a distillation residue.

Further objects and advantages of our invention will be apparent to those skilled in the art as the description thereof proceeds.

We have discovered that it is possible by emplaying the new and novel type azeotrope formers hereinafter disclosed, to reverse the order of azeotrope formation as obtained in the conventional azeotropic distillation of complex hydrocarbon fractions. In the application of azeotropy to petroleum fractions a great number of azeotrope formers have been employed which exert a change in the vapor pressure relationship existing in said hydrocarbon fraction in such a manner that the lower boiling azeotropes are formed with the paraflln hydrocarbons followed by the naphthenic, olefinic, dioleflnic and finally'aromatic hydrocarmatic hydrocarbon as azeotroplc bottoms.

bons. In this manner the distillation of a complex petroleum fraction in the presence of such an azeotrope former consists in taking overhead initially an aneotropic mixture comprising predominately paraffln hydrocarbons with the specific aneotropic former employed, continued distillation in the presence of said azeotrope former gives an azeotropic overhead comprising predominately naphthenic hydrocarbon together with the azeotrope former, and subsequently in the same manner aaetropic mixtures oi oleflnlc and dioleilnic hydrocarbons with said azeotrope former may be taken overhead leaving the agoor purposes of this disclosure the hydrocarbon types as contained in any particular fraction will be designated as relatively non-aromatic or relatively aromatic depending upon their position in the above series. Thus in the discussion of conventional azeotropy above the ease of azeotrope formation with the azeotrope formers employed pr from the relatively non-aromatic to the relatively aromatic hydrocarbons.

We have been able by our process to reverse the above order and thereby to take overhead initially in an azeotropic distillation of a complex hydrocarbon fraction an azeotropic mixture of the relatively more aromatic hydrocarbons contained in the fraction together with the azeotrope former of the type we have discovered. We have found this unusual procedure to be possible by employing an azeotrope former consisting oi a naphthenic hydrocarbon preferably boiling below but not more than 9 0. below the relatively more aromatic component contained in said complex hydrocarbon fraction. Thus in the azeotropic distillation oi. a complex petroleum fraction employing an azcotrope former of the type described above we are able to obtain as our initial azeotropic distillate a minimum boiling mixture of the aromatic component of said fraction together with said aseotrope former and in successive stages azeotropic mixtures of dioieflnic, oleflnic and naphthenic hydrocarbons with the azeotrope former employed.

In the operation of our process we may employ for example, a petroleum traction oi relatively narrow boiling range containing a fraction of 1% to about 10% by volume or more of an aromatic hydrocarbon as well as paraiiinic, napht'henic, and oleflnic hydrocarbons from which it is desired to remove or to recover said aromatic hydrocarbon. This fraction is subjected to controlled distillation in the presence 0! an added naphthenic material boiling below but not more than 9 C. below the specific aromatic hydrocarbon, and more fully disclosed hereinafter. The efiect of such a distillation is to give an overhead product consisting of substantially all of the aromatic hydrocarbons contained in the fraction as an aseotropic mixture with the above naphthenic compound leaving as azeotropic bottoms substantially all of the relatively non-aro matics contained in the above fraction which may or may not contain small percentages of the azeotrope former employed. By suitable modification at the above described process we are also able to separate substantially pure olefin or naphthene fractions by means of side cuts in the above distillation or by secondary distillation of the primary aneotropic bottoms in the presence of an additional amount of our aseotrope former. The recovery oi the aromatic. oleflnic or naphthenic compounds iron their aseotropes may be accomplished in our by any desirable means 4 such as a secondary areotropic distillation with one of many conventional aaeotrope formers whereby the naphthenic azeotrope former employed in the primary azeotropic distillation may be separated as a secondary azeotropic distillate leaving substantially all the aromatic in relatively pure form as secondary azeotropc bottoms.

Thus, in the recovery of benzene contained in the amount of about 5% in a petroleum traction boiling in'the range of to 200 F. by means of our process, a suitable quantity of methylcyclopentane is added to the hydrocarbon fraction and the mixture subjected to a controlled distillation whereby an azeotrope of methylcyclopentane and substantially all of the benzene is obtained as an overhead product which may or may not contain small amounts of lower boiling nonaromatic hydrocarbons. In this connection we have found that the presence of hexane in the fraction is not objectionable inasmuch as an azeotrope is formed between benzene and hexane boiling at about 683 C. and therefore, is recovered with the methylcyclopentane benzene areotrope. The benzene may be recovered from this azeotropic distillate by subjecting the mixture to controlled fractional distillation in the presence of a secondary azeotrope former such as acetone whereby the methylcyclopentane and any lower boiling non-aromatics are taken overhead as an azeotroplc mixture with the acetone. leaving as azeotropic bottoms essentially pure benzene.

It is within the spirit and scope of our invention to employ any desired method in separating the azeotropic mixture of aromatic, oleflnic, or naphthenic hydrocarbons with our particular azeotrope formers such as for example solvent extraction, extractive distillation, azeotropic distillation and the like.

We have also found this process to be applicable to the separation of sulfur type compounds such as thiophene, methylthiophene, dimethylthiophene, and the like which are ofttimes contained as undesirable contaminants in petroleum fractions. It is also within the scope of our invention to employ this process for the purification of raw mixtures of thiophene and other compounds occurring as intermediate products in the preparation of thiophene and its homologs.

We do not wish to limit ourselves by the above description of our invention inasmuch as we may employ it on any desired hydrocarbon fraction using a suitable naphthenic type aaeotrope former. Thus, we may separate toluene from a petroleum fraction in which it is contained by azeotropicaily distilling said fraction in the presence of an azeotrope former of the type hereinbefore disclosed such as for example ethylcyclopentane. In like manner we may separate by means of our invention xylenes, cumenes or other high boiling aromatics contained in petroleum stocks.

Besides methyl and ethylcyclopentane mentioned other alkylated cyclopentanes may be used for effecting separations wherein the relatively aromatic hydrocarbons are distilled overhead as azeotropes with the alkylated cyclopentanes. These are selected according to th lreiative boiling points of the alkylated cyclo tanes and the stock. Additionally other naphtlienesor naphthenlc homologs may be used in the process depending on the stock to be treated. These include 7 the cyclopropanes, cyclobutanes. cyciohexanes, cycloheptanes and the like. As described herein the preferred aseotrope formers are those naphthenes which boil below but not more than asmsos 5 9 0. below the aromatic hydrocarbon which is to be azeotroped.

The novelty and the advantages 01 our invention become immediately apparent to those skilled We have employed for the separation or one hydrocarbon from another hydrocarbon a third hydrocarbon; the aaeotrope former consisting oi a third hydrocarbon rather than a diflerent type organic compound TABLE 1 Direct azeotropina with acetone-Case I I Distillate Parts Type Total I Acetone cyelohcxanc Distillation, Azeotroplc 2m 95 317 Extraction, water e- Extract 817 parts with water Distillation, simple an 222 Total distillate, 539 parts. Total to extract, 317 parts.

Azeotroping with methulcyciopentanease II I namaa Pam y Methy} TONI Acetone cyclopentlnc Distillation, Azeotropic i5 6 50 Distillation, Aleotrpic 55 46 liD Extraction, water Extract 100 parts with water Distillation, simple 55 Total distillate, 205 parts.

Total to extract, 100 parts.

Inspection of these data shows that case I would require the distillation 01' 539 barrels of overhead and the extraction of 317 barrels of distillate to remove the barrels oi benzene, while case H would require the distillation 0! only 205 barrels of overhead and the extraction of only 100 barrels of distillate to remove the same quantity of benzene. The use of methylcyclopentane therefore decreases the distillation load by approximately 334 barrels or 62%, and the extraction load by approximately 217 barrels or 68%. Furthermore, this advantage is incurred in any application of our process for the extrac-. tion 01' relatively small amounts of hydrocarbon components.

The following example of out in our laboratories will only the economical manner in centages oi aromatics may petroleum fractions but also accrue as a result of the matics from said fraction.

Percent Benzene l0 Cyclohexane 70 Dimethylpentane (3,8 and 2,3) 15 Other non-aromatic hydrocarbons 5 In order to more fully understand the process of our invention the boiling points oi the individual components in the fraction and the azeotropes formed therein are given below:

Component Dom. oi

Benzene m. 1 c an. 7 11.5 2, or 8,3-dimnthylpentanc 86 to 79 to 8| by the formation of closely azeotropes with each, Thus, we may have in a given traction two reasons for the removal 01' aromatic hydrocarbon contained therein. remove the aromatic hydrocarbon and recover it plates. This mixture was subjected to a controlled fractional distillation whereby an azeotrope of methylcyclopentane and benzene was taken overhead at a temperature or approximately 71.5 C. The benzene, methylcyclopentane aseotrope consists of 90% methylcyclopentane and 10% benzene and The overhead product from ascotropic distillation was obtained at 51.4 C. and consisted of an azeotropic mixture of 10 volumes of methylcyclopentanc and volumes oi' acetone volume of essentially pure cyclopentane-acetone aseotrope from ary aaeotropic distillation was of water washing, the

as'msos from the water by simple distillation. Employing a feed stock such as the one herein described containing small amounts of lower boiling aromatics, it may be necessary for the recovery of the methylcyclopentane azeotrope former to sublest the extract of the above water washing to a simple distillation to separate said low boiling non-aromatics from the methylcyclopentane.

The azeotropic bottoms from the primary azeotropic distillation consisted of 7 volumes of cyclehexane and 2 volumes of dimethylpentane and higher boiling non-aromatics. Due to the absence of benzene azeotropes in these bottoms it was possible to obtain by means of simple fractional distillation cyclohexane of 96% to 97% purity. Any 2,2,3-trimethyibutane boiling point 80.8 0., 2,4-dimethylpentane boiling point 805 0., or 2,2-dimethylpentane boiling point 18.9 contained in the original fraction would remain as impurities in the cyclohexane, but apparently these compounds are present in petroleum distillates in very small percentages and do not prevent the production of cyclohexane by the above process of more than 95% purity.

The example described above is not meant to limit our invention as it is within the spirit and the scope of this invention to perform a separation of any of the hydrocarbon components hereinbefore listed from fractions in which they are contained, and it is also within the scope of this invention to separate sulfur compounds in the same manner from petroleum fractions in which they are contained.

The foregoing description of our invention is not to be taken as limiting our invention but as only illustrative thereof, since many variations may be made by those skilled in the art without departing from the spirit and scope of the followin claims.

We claim:

1. A process for the treatment of a complex hydrocarbon fraction comprising aromatic components and non-aromatic components having from 6 to 9 carbon atoms to separate one component from the other components contained therein which ordinarily distill from said hydrocarbon fraction in the same temperature range as said component distills therefrom, which comprises azeotropically distilling said complex hydrocarbon fraction in the presence of a sufflcient amount of an added azeotrope former consisting of a naphthene boiling below, but not more than about 9 C. below, said aromatic components to vaporize the aromatic components contained in said fraction together with said added azeotrope former as a minimum boiling azeotropic mixture thereby leaving the non-aromatic components as distillation bottoms.

2. A process for the treatment of a complex hydrocarbon fraction containing an aromatic hydrocarbon having from 6 to 9 carbon atoms together with a mixture of non-aromatic hydrocarbons to separate said aromatic hydrocarbon from the non-aromatic hydrocarbons contained therein which ordinarily distill from said hydrocarbon fraction in the same temperature range as said aromatic hydrocarbon distills therefrom, which comprises azeotropically distilling said complex hydrocarbon fraction in the presence of a suiiicient amount of an added azeotrope 70 former consisting of a naphthenic hydrocarbon boiling below, but not more than about 9 C. below, said aromatic hydrocarbon to vaporize the aromatic hydrocarbon together with said added 8 mixture thereby leaving non-aromatic hydrocarbons as distillation bottom.

3. A process for the treatment of a complex hydrocarbon fraction containing benzene as well as oleflnic, naphthenic and parafiinic hydrocarbons to separate the benzene from the other components contained therein which ordinarily distill from said hydrocarbon fraction in the same temperature range as the benzene prohibiting its separation by simple fractional distillation, which comprises azeotropically distilling said complex hydrocarbon fraction in the presence of a sumcient amount of added methylcyclopentane to vaporize said benzene together with said added methylcyclopentane as a minimum boiling azeotropic mixture thereby leaving non-aromatic hydrocarbons as distillation bottoms.

4. A process for the treatment of a complex hydrocarbon fraction containing toluene as well as oleilnlc, naphthenlc and paraflinic hydrocarbons to separate the toluene from the other components contained therein which ordinarily distill from said hydrocarbon fraction in the same temperature range as the toluene prohibiting the separation of said toluene by means of simple fractional distillation, which comprises azeotropically distilling said complex hydrocarbon fraction in the presence of a sumcient amount of added ethylcyciopentane to vaporize said toluene together with said added ethylcyclopentane as a minimum boiling azeotropic mixture thereby leaving the non-aromatic hydrocarbons as distillation bottoms.

5. A process for the treatment of a complex hydrocarbon fraction containing an aromatic hydrocarbon having from 6 to 9 carbon atoms to separate one component from the other components contained therein which are ordinarily separable by simple fractional distillation but may not be separated in the presence of said aromatic component due to the formation of closely boiling azeotropes with said aromatic components, which comprises removing said aromatic component from the fraction by azeotropically distilllng said complex hydrocarbon fraction in the presence of a sufiicient amount of an added azeotrope former consisting of a naphthene hydrocarbon boiling below, but not more than about 9 C. below said aromatic hydrocarbon to vaporize said aromatic hydrocarbon together with said added azeotrope former as a minimum boiling azeotropic mixture thereby leaving non-aromatic hydrocarbons as distillation bottoms and distilling said bottoms in the absence of the aromatic hydrocarbon to yield pure components.

6. A process for the preparation of substantially pure cyclohexane from a complex hydrocarbon fraction containin benzene as well as cyclohexane which fraction is impossible of separation in the presence of said benzene due to the formation of closely boiling azeotropic mixtures of benzene and cyclohexane and benzene and other non-aromatic hydrocarbons present in said fraction, which comprises azeotroplcally distilling said complex hydrocarbon fraction in the presence of a sufficient amount of added methylcyclopentane to vaporize said benzene together with said added methylcyclopentane as a minimum boiling azeotropic mixture thereby leaving the non-aromatic hydrocarbons present in said complex hydrocarbon fraction as distillation bottoms which may then be separated in the absence of benzene by means of simple fractional distillation to yield substantially pure cyclohexaseotrope former as a minimum boiling azeotropic one.

'7. A process for the treatment 01 a complex hydrocarbon fraction containing benzene as well as naphthenic and parafllnic hydrocarbons to separate the benzene from the other hydrocarbon components contained therein which ordinarily distill from said hydrocarbon fraction in the same temperature range as the benzene prohibiting its separation by simple distillation, which comprises azeotropically distilling said complex hydrocarbon fraction in the presence 01' a sunlcient amount of added methylcyclopentane to vaporize said benzene together with said added methylcyclopentane as a minimum boiling azeotropic mixture thereby leaving the non-aromatic hydrocarbons as distillation bottoms.

8. The process as set forth in claim 1 in which the naphthene is an alkyl cyclopentane.

9. The process as set forth in claim 1 in which the complex hydrocarbon fraction contains xylene as well as olennic, naphthenic and parafflnic hydrocarbons.

10. The process as set forth in claim 3 in which the complex hydrocarbon fraction contains cyclohexane, benzene, 3,3 and 2,3 dimethyi pentane.

11. The process as set forth in claim 5 in which the complex hydrocarbon fraction is of narrow boiling range and includes a naphthenic hydrocarbon boillng in the range or an aromatic compound contained in said fraction which aromatic compound prevents separation of said naphthenic hydrocarbon by the formation or azeotropes within the traction of said aromatic hydrocarbon with the naphthenic and paraillnic hydrocarbons, which azeotropes have small boiling point dill'erences.

GEORGE R. LAKE. J H. S'I'RIBIEY.

REFERENCES CITED The following relerenices are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,892,772 Jaeser et a1 Jan. 3, 1933 2,371,860 Walls et a1. Mar. 20, 1945 2,382,446 Ross et al Aug. 14, 1945 OTHER REFERENCES 

